President Jaafar Nimeri formally took the Sudan into a fledgling Egyptian-Syrian joint political command today, in a move designed primarily to bolster his government against Libyan efforts to overthrow it.

President Answar Sadat of Egypt and Hafez Assad of Syria left their prestige to the Nimeri government, expanding the joint command to include more than half the Arab world's population.

This framework for a loose confederation was established in December. It was intended more to symbolize the end of the more than year - old rift between Damascus and Cairo than to confront Israel.

Absent from the two-day summit here, but vitally concerned by what diplomats described as largely a public relations operation for Nimeri, was silent partner Saudi Arabia, Sudan's oil-rich neighbor across the strategic Red Sea and a major investor here.

The Saudi and Egyptian governments feel directly threatened by what they perceive as Libya's continuing efforts to topple Nimeri. The effort almost succeeded last July in a bloody but unsuccessful coup.

Dependent on a friendly power in Sudan for its lifelines - the Nile River waters - Egypt has had its own troubles with Libya and formed a defense pact with Nimeri after the coup attempt in Khartoum. The Egyptian food riots last month provided another reason for Sadat to show his enemies that he has friends in Syria and the Sudan.

(United Press International reported from Cairo that Egypt accused Quaddi of launching a "new criminal scheme" in the explosion of a bomb that wounded nine persons Saturday in the Mediterranean port, Alexandria.)

Assad's presence here reflected Syria's growing role in the Middle East and new willingness to spread its influence far afield from its immediate neighbors, Jordan and Lebanon.

Syrian Information Minister Ahmed Iskandar told reporters that the summit meeting here was aimed at "assuring the entire world that Egypt and Syria are backing Nimeri and his regime - against any challenge."

Neither Egypt nor Syria, however, appeared to be willing to back Sudanese efforts to castigate by name Libya or the Soviet Union which Nimeri is known to feel is supporting Col. Muammar Qaddafi's government in Tripoli.

This reluctance was evident in the summit's joint communique, which discussed in only the most general terms the sensitive question of Red Sea security.

Near vital world oil trade routes, with its entrance at the straits of Bab El Mandeb, a choke point for shipping bound for the Israeli port of Eilat, the Red Sea area is the scene of growing concern.

The long Eritrean separatist rebellion continues against a weakening Ethiopian government. The coming independence of the French territory of Djibouti, now Ethiopia's only workable outlet to the sea, has raised fears of further upheaval. Both Somalia and Ethiopa covet the strategically located territory.

Although the communique urged that the Red Sea be considered a "lake of peace," implying the desired absence of great power rivalries in the area, no nation was singled out for citicism.

Syria has aided the Eritrean rebellion since it began in the early 1960s, while Nimeri recently has aided the rebels openly to pay back the Ethiopians for accepting Libyan aid and harboring Sudanese opposition forces.

Cairo is about to play host to an Afro-Arab summit conference and wants to avoid angering black Africans, who identify with Ethiopia, by openly backing the largely Arab-supported Eritrean rebellion.

Egypt hopes the Cairo conference will provide continued black African support for the Arabs' campaign to force Israel into major concessions at resumed Middle East peace negotiations in Geneva.

Reflecting Egyptian caution was Foreign Minister Ismail Fahmi, who bantered with reporters rather than answering their questions about Saudi and Sudanese press reports alleging Israeli use of Ethiopian naval bases.

Instead, he tactfully said the Red Sea was "almost an Arab lake" and suggested that there "must be more coordination and harmony" among the bordering states.